1. Field of the Invention
Coffee makers of the kind referred to herein are adapted to prepare a fresh coffee beverage fully automatically. Such coffee makers comprise a coffee bean container for receiving coffee beans to be ground, a grinder unit for grinding the coffee beans into ground coffee powder, and a brewing unit to which the freshly ground coffee powder is fed. In the brewing unit, the coffee powder is compressed and, thereafter, hot pressurized brewing water flows through the compressed coffee powder to extract a coffee beverage. Such coffee makers are also called “Espresso Coffee Machines” in general language use.
There is one particular problem in connection with these coffee makers, inasmuch as a lack of coffee beans in the grinder unit should be detected as soon as possible; otherwise, the drive motor of the grinder unit could overheat if it runs under no load conditions for a certain period of time. Another problem is seen in the fact that a relationship should be established to the amount of coffee beans already ground in the actual grinding cycle once the lack of coffee beans has been detected; thereby, after refilling the coffee bean container with coffee beans, the grinding cycle can be continued to grind only the remaining amount of coffee beans required to complement the already ground amount such that the correct total amount of coffee powder results that is required to brew a proper coffee beverage. Otherwise, the already ground amount of coffee would have to be discarded without having been used and replaced by a new, full charge of coffee powder.
2. Description of the Related Art
The detection of a lack of coffee beans in a grinder unit of a coffee maker by means of conventional sensing means is problematic, since these are subject to rapid contamination, the more so as coffee beans are fatty and because coffee dust develops during the grinding operation, that dust being destructive for most of the sensing means used up to now.
Moreover, it is known in the prior art to measure the current the electric motor driving the grinding unit draws and to conclude from the magnitude of the current to the presence or absence of coffee beans in the grinder unit. Such a method may be theoretically possible since the motor of the grinder unit draws more current under load than during operating in a no-load condition. However, this method did not prove its reliability in practice because, in most cases, a gearbox is arranged between motor and grinder unit, hampering the detection of a lack of coffee beans because, due to the high reduction ratio of the gearbox, only a small difference in current draw can be observed when the motor is operated under load and under no-load conditions, respectively.
A further problem is presented by the fact that certain components of the grinder unit run in over the months and years, resulting in a continuous reduction of the current draw of the motor; thus, the detection of a lack of coffee beans by means of measuring the current is additionally complicated and hampered, respectively.
Finally, further parameters as, for instance, the ambient temperature, the ambient humidity, the kind, size and degree of roasting of the coffee beans to be ground, the selected degree of grinding as well as changes and wear of the grinder unit due to aging can have an influence on the current draw and, thereby, on the validity of the measurement.